LibJS: Disable bytecode optimizations by default

The optimization passes are not stable, which makes test262 flaky.
Address this by introducing a new OptimizationLevel::None and making it
the default.

This removes all the flakiness from test262 in my testing.

We can enable optimizations by default again once they have been made
stable. :^)

Userland/Libraries/LibJS/Bytecode/Interpreter.h

@@ -66,8 +66,10 @@ public:
     Executable const& current_executable() { return *m_current_executable; }
 
     enum class OptimizationLevel {
-        Default,
+        None,
+        Optimize,
         __Count,
+        Default = None,
     };
     static Bytecode::PassManager& optimization_pipeline(OptimizationLevel = OptimizationLevel::Default);